The invention relates to a method and apparatus for selective catalytic oxidation of carbon monoxide in a mixed gas stream containing hydrogen, in which an oxidizing gas is added to the mixed gas stream and the latter is conducted through a reactor containing the catalyst.
Methods for selective catalytic oxidation of carbon monoxide in a mixed gas stream which also contains hydrogen are used for example in conjunction with catalytic ammonia production, as disclosed in German patent document DE-OS 29 30 214, or in the preparation of hydrogen as a fuel for a fuel cell, as disclosed in International patent document WO 93/19005. Numerous methods and devices with suitable catalysts for such selective oxidation of carbon monoxide are already known, including in addition to the above documents, German patent document DT 15 67 492 and Japanese patent document JP 3-208801 (A). In the latter publication, oxidation takes place in two parallel reactors to which the mixed gas stream and the oxidizing gas are added alternately and intermittently, with one reactor adsorbing carbon monoxide while the previously adsorbed carbon monoxide is oxidized in the other. In the method disclosed in the above-mentioned German patent document DE-OS 29 30 214 A1, the oxidizing gas, (air in that case) is mixed with the mixed gas stream containing hydrogen and carbon monoxide, upstream of the carbon monoxide oxidation reactor, and is conducted together with the latter into the reactor.
The above-mentioned International patent document WO 93/129005 discloses a single- or multistage reactor for selective catalytic carbon monoxide oxidation in which each stage in turn can consist of one or more parallel reactor units. A mixed gas stream containing primarily hydrogen as well as small quantities of carbon monoxide (for example, from a hot steam reformation of a hydrocarbon, such as methanol) is supplied on the input side. As an alternative to admixture of air or oxygen to the mixed gas stream upstream of the carbon monoxide oxidation reactor, provision is made for conducting the oxidizing gas directly into the reaction chamber. The gas stream from the carbon monoxide oxidation reactor, which is purified of carbon monoxide and consists entirely of hydrogen, is conducted as a fuel to a fuel cell. Removal of the carbon monoxide prevents harmful poisoning of the fuel cell. To perform selective catalytic carbon monoxide oxidation, a two-stage process is provided, with a higher reaction chamber temperature of approximately 160.degree. C. in the first stage and a lower reaction chamber temperature of less than 105.degree. C. in the second stage. To adjust these temperatures, the catalyst bed reaction chambers are traversed by cooling coils of the respective cooling circuits with controllable coolant throughput volumes.
German patent document DE-OS 43 34 983 A1 describes a method for removing carbon monoxide for a hydrogen-rich gas in at least two stages, including CO-oxidation stages and/or methanization stages. CO oxidation is accomplished by supplying air to a selective oxidation catalyst containing a noble metal, for example, Pt/Al.sub.2 O.sub.3 - , Ru/Al.sub.3 O.sub.3 - or Pt zeolite material.
German patent document DE-OS 4334 981 A1 utilizes a reactor for catalytic removal of carbon monoxide from a hydrogen-rich gas with a selective CO-oxidation catalyst containing a noble metal. The catalyst has a structure which is coated with a catalyst, and produces a turbulent flow, favoring heat transport.
The object of the present invention is to provide a method and apparatus of the type described above, in which the oxidation reaction can be suitably controlled by the reaction path of the mixed gas stream, and in which pre-cooling of a mixed gas stream coming from a hot steam reformation is not required.
This object is achieved according to the invention, by conducting a controlled or regulated throughput volume of the oxidizing gas into the CO-oxidation reactor, so that heat generation in the exothermal CO-oxidation reaction can be influenced. In combination with a regulation of the coolant volume flow, therefore, the temperature level of the CO-oxidation stage can be adjusted. By passive cooling of the mixed gas stream with the aid of static mixing structures, the temperature of the mixed gas stream can be set to a desired temperature level even before it reaches the active reaction volume. This arrangement offers, among other things, the advantage that when the reformate from a methanol hot-steam reformation is used as the input gas mixture, intermediate cooling of the reformat prior to introduction into the CO-oxidation reactor, as is frequently provided in conventional installations, is not absolutely necessary. Instead, the reformate can be conducted directly into the CO-oxidation reactor. Therefore, reformate cooler can either be eliminated altogether, or in any event made much smaller than usual. There is also no need for an additional complex heating circuit to produce the required reactor temperature.
The method according to the invention permits an extremely flexible process, and hence a comparatively high space velocity for the mixed gas stream. The resultant flexible controllability of process guidance makes the method especially suitable for mobile applications, for example in motor vehicles powered by fuel cells.
A feature of the invention is that it can be used advantageously in systems in which hydrogen for fueling a fuel cell is obtained by methanol reformation. When this method is used, intermediate reformate cooling is eliminated. Moreover, because the heat-carrying medium for cooling the CO-oxidation reactor is kept at the fuel cell temperature, the temperature gradient between the hot-steam reformation process and the fuel cell is utilized in the intermediate CO-oxidation stage and thus can be controlled to achieve a high degree of CO oxidation.
The provision of a static mixing structure in the input area of the CO-oxidation stage promotes uniform distribution and mixing of the mixed gas stream. Particularly when the oxidizing gas is added upstream of the CO-oxidation stage, the static mixing structures causes a mixing of the oxidizing gas with the mixed gas stream containing the hydrogen. In addition, the contact between the mixed gas stream and the outside wall of the CO-oxidation reactor is increased and thus passive cooling of the mixed gas stream is achieved before it reaches the active reaction volume.
According to another embodiment of the invention, a compact, modular-design plate reactor is provided for selective catalytic CO oxidation, whose geometry can be adapted easily to the other system components and is especially suitable for mobile applications in motor vehicles powered by fuel cells. The modular plate reactor design makes it possible, in conjunction with the possibility of adding the oxidizing gas in adjustable amounts along the reaction path, to have a high space velocity for the mixed gas stream, which in turn makes it possible to design the reactor as a whole with comparatively small volume and weight.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.